Press Releases & Publicity

Does anyone know what the best way to go about sending out press releases is ?

I'm guessing that any article or announcemnet sent must be newsworthy and not a product advertisement. That being the likely case, how does one write a release that would be valuable for the Press (what subjects)?

Also, I would anticipate that sending out releases should be handled by a firm that does this full time, which, of course, means a significant cost involved.

Any ideas are appreciated. Thank you.